1. Field of the Invention
The present invention relates to a flux for soldering used when soldering various electronic components on a printed circuit board, and more particularly to a flux used when soldering is performed onto copper lands of a printed circuit board subjected to electro less nickel plating, as well as a soldering method and a printed circuit board.
2. Description of Related Art
When electronic component s are soldered onto a printed circuit board, soldering onto copper lands formed on the printed circuit board has conventionally been performed with the use of tin/lead alloy solder or lead-free solder. In order to prevent oxidation of copper, electro less nickel plating is often applied to the surfaces of the lands.
However, when electro less nickel plating is applied to the lands, a trace quantity of phosphorous compound remains in a nickel plating layer because hypophosphite is used as reducing agent.
Therefore, during soldering with the use of solder alloy to the surface of electro less nickel plating, the nickel in the nickel plating diffuses into the melted solder alloy, and phosphorus segregates locally at the boundaries between the nickel plating layer and the solder alloy. This causes extremely concentrated portions of phosphorous, and hence bonding strength is lowered thereby to strip soldering in some cases.
Japanese Unexamined Patent Publication No. 6-264284 discloses a method of forming a wiring board including: forming electro less nickel/boron plating film on a wiring pattern composed of tungsten or molybdenum and on a connecting pad which are formed on a ceramic printed wiring board; performing heat treatment; and forming a nickel/phosphorous plating film. In this method, the drop in bonding strength, due to concentration of phosphorus in connecting interface during soldering and brazing of gold/tin, etc., is prevented by setting phosphorus content at not more than 6%, and a plating film at 0.3 to 2 μm.
Japanese Unexamined Patent Publication No. 10-163404 discloses an input/output terminal for BGA formed by attaching, instead of conventional solder balls, Cu balls whose surface is coated with a solder plating layer, to a Cu pad whose surface is coated with an electro less Au plating layer via a P-containing electro less Ni plating layer.
These methods require additional processing such as two-stage plating operation and ball processing. Additionally, the use of specific board and balls lowers versatility, at which the present invention aims from the point of view of performing general solder connection on a general board.